This invention relates to a process for producing both electrolytes for a redox cell.
Since electric power can be readily converted into various types of energy to be controllable and to have no environmental pollution at consumption, the ratio of the electric energy consumption to total energy consumption is increasing in every year. The feature of supplying electric power resides in the simultaneous production and consumption. It is required for electric power industry to supply electric power of high quality at constant frequency and voltage while rapidly responding to the variation in the power consumption in the above restriction. The output power is in fact hardly varied, but a nuclear power plant and a modern coal power plant of high efficiency are operated as high as the rated maximum efficiency, and a hydraulic power plant adapted to generate in response to the variation in the power consumption is used for an increase in the large power demand during a day time.
Thus, the night excessive power by the nuclear and coal power plants of high economy is stored with pumped-hydro electric plants, but the installation conditions of the pumped hydroelectric plants gradually become severe.
From the above mentioned circumstances, various types of secondary battery have been studied as a process for storing electric power of high universal energy without environmental pollution, and the redox cell in which two kinds of redox solutions are contacted through an ion exchange membrane has been recently developed remarkably.
The principle of this redox cell will be described with reference to FIG. 1, which shows an electric power storage system using a 2-tank type redox cell system. In FIG. 1, numeral 1 designates a power plant, numeral 2 designates a substation, numeral 3 designates a load, numeral 4 designates an inverter/connector, numeral 5 designates a redox cell system, and the redox cell system 5 is composed of tanks 6, 7, a flow type electrolytic cell 8 and so on.
The electrolytic cell 8, partitioned by a ion exchange membrane, is provided therein with an anolyte chamber 10a and a catholyte chamber 10b. The chamber 10a contains an anode 11 and an anolyte such as hydrochloric acid solution including, for example, Fe ions, while the chamber 10b contains a cathode 12 and a catholyte such as hydrochloric acid including, for example, Cr ions. A pump 13a is provided between the tank 6 and the chamber 10a, an anolyte circulation passage 14 is formed between the tank 6 and the chamber 10a, a pump 13b is provided between the tank 7 and the chamber 10b, and a catholyte circulation passage 15 is formed between the tank 7 and the chamber 10b.
In the constitution thus constructed, electric power generated at the power plant 1 and transmitted to the substation 2 is transformed to a proper voltage, and supplied to the load 3.
On the other hand excessive power is produced at night, the power is converted from AC to DC by the converter 4, and electric energy is stored in the redox cell system 5. In this case, the redox cell system is charged while circulating with the pumps 13a, 13b, the anolyte and the catholyte through the chambers 10a and 10b. When the supplied power is less than the demand power, the DC power is generated while circulating the anolyte and the catholyte through the chambers 10a and 10b with the pumps 13a, 13b, and is converted from DC to AC by the inverter 4. Thus, the power is supplied through the substation 2 to the load 3. In the redox cell system Fe ions are used for the anolyte and Cr ions are used for the catholyte. The charge and discharge reactions in the cell 8 are shown as the following formulae (1) to (3): ##STR1##
The power storage system using the redox cell is as described above. Since chromium solution used as a catholyte of the electrolyte of the redox cell has employed electrolytic chromium having 99.5% of purity, it is expensive and accordingly a large obstruction to use the redox cell in industry. Thus, a process for dissolving ferrochromium containing chromium and iron in hydrochloric acid and a process for dissolving finely pulverized ferrochromium ore in hydrochloric acid to be employed directly as the electrolytes of the redox cells have been proposed (Japanese Patent Application No. 4183/84).
Further, the process for dissolving the ferrochromium in hydrochloric acid has been also proposed to dissolve the ferrochromium, then to oxidize iron and chromium, to extract chriomic chloride (CrCl.sub.3) and ferric chloride (FeCl.sub.3) in solvent and to refine it (Japanese Patent Publication No. 37530/82 official gazette, Japanese Patent Laid-open No. 137895/75 official gazette).
The process disclosed in Japanese Patent Application No. 4183/84 dissolves ferrochromium and/or chromium ore in hydrochloric acid and extracts chromium(III) and iron(II).
However, the process has such disadvantages that slightly dissolves chromium ore in the hydrochloric acid and the solubility of relatively inexpensive high carbon ferrochromium in hydrochloric acid is low so that the process must be done at high temperatures for a long period and low carbon ferrochromium is rather expensive and another disadvantage that gas bubbles are generated by the influence of silicon and carbon contained.
Moreover, since low carbon ferrochromium has high hardness and ductility, it can be hardly pulverized to fine particles. Its solubility is superior to the high carbon ferrochromium, but there also arises another disadvantage that the process for pulverizing requires a long period.